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United States Patent |
6,086,353
|
Klaus
|
July 11, 2000
|
Two-stage electric injection unit with rotating plunger
Abstract
The invention is directed to a two-stage, all-electric injection unit. More
specifically, an injection unit in which the feed screw is preferably used
only for plastication and the injection of the plasticized material is
accomplished by a separate accumulator having an electrically driven
linear actuator, such as a ball screw mechanism. The plunger of the
accumulator is configured with a helical flight and is rotatable by way of
a one-way clutch interposed between the plunger and the ball screw.
Inventors:
|
Klaus; M. Barr (Cincinnati, OH)
|
Assignee:
|
Cincinnati Milacron Inc. (Cincinnati, OH)
|
Appl. No.:
|
024731 |
Filed:
|
February 17, 1998 |
Current U.S. Class: |
425/145; 425/557; 425/558; 425/560 |
Intern'l Class: |
B29C 045/70 |
Field of Search: |
425/145,557,558,560
|
References Cited
U.S. Patent Documents
3861841 | Jan., 1975 | Hanning | 425/146.
|
4290701 | Sep., 1981 | Schad | 366/77.
|
4722679 | Feb., 1988 | Farrell | 425/146.
|
4734243 | Mar., 1988 | Kohama et al. | 264/328.
|
4758391 | Jul., 1988 | Shimizu et al. | 264/40.
|
5281384 | Jan., 1994 | Banks | 264/297.
|
5454995 | Oct., 1995 | Rusconi et al. | 264/328.
|
5606707 | Feb., 1997 | Ibar | 425/144.
|
5863567 | Jan., 1999 | Klaus | 425/557.
|
Primary Examiner: Heitbrink; Tim
Attorney, Agent or Firm: Friskney; Stephen H.
Claims
What is claimed is:
1. An injection molding machine including an injection unit comprising:
a feed screw capable of rotational movement within a barrel having an inlet
and an outlet,
rotational drive means including a variable speed electric motor, for
rotating the feed screw within the barrel in order to plasticize and
convey material from the barrel inlet to the barrel outlet,
a melt accumulator having a plunger received within a cylindrical chamber
connected to receive plasticized material from the barrel outlet; and
plunger drive means including a mechanism driven by an electric motor, for
imparting translational and rotational movement to the plunger of the
accumulator, thereby controlling the position of the plunger in the
cylindrical chamber of the accumulator.
2. The injection molding machine of claim 1 wherein the mechanism of the
plunger drive means comprises a ball screw mechanism coupled to the
plunger of the accumulator.
3. The injection molding machine of claim 1 wherein the mechanism of the
plunger drive means comprises a roller screw mechanism coupled to the
plunger of the accumulator.
4. The injection molding machine of claims 2 or 3 wherein the mechanism of
the plunger drive means further comprises a one-way clutch interposed
between the screw mechanism and the plunger of the accumulator.
5. The injection molding machine of claim 1 wherein the plunger of the
accumulator is generally cylindrical in shape and is capable of traversing
a defined length of stroke, such that the ratio of the stroke of the
plunger to the diameter of the plunger is at least five.
6. The injection molding machine of claim 5 wherein the ratio of the stroke
of the plunger to the diameter of the plunger is between ten and fifteen.
7. The injection molding machine of claim 1 wherein at least a portion the
plunger of the melt accumulator has a helical flight.
8. The injection molding machine of claim 7 wherein the mechanism of the
plunger drive means comprises a screw mechanism coupled to the plunger of
the accumulator.
9. The injection molding machine of claim 8 wherein the mechanism of the
plunger drive means further comprises a one-way clutch interposed between
the screw mechanism and the plunger of the accumulator.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to molding machines and, more
particularly, to a two-stage electric injection unit for an injection
molding machine.
2. Description of the Related Art
The injection unit of an injection molding machine provides essentially two
functions during the course of a normal cycle of operation; namely,
injection and extruder. In a standard reciprocating screw injection
molding machine, the extruder function is accomplished when the screw is
rotated, gradually moving plastic melt toward the forward end of the
screw, thereby creating a pressure or force to move the screw rearward to
its pre-injection position as the melt accumulates. When a sufficient
amount of material is accumulated ("a shot"), the screw is moved rapidly
forward (without rotation) to inject the melt straight into the mold, thus
performing the injection function.
The injection unit of a molding machine can also be designed as a
"two-stage" system where the extruder and injection functions are
performed by separate machine elements. In a two-stage injection system,
the extruder or plasticizing function is still performed by a feed screw
in a heated barrel, but the plastic melt is diverted into an "accumulator"
(usually positioned adjacent the plasticizing barrel) rather than being
conveyed from the barrel directly to the mold cavity. The accumulator is
subsequently operated to perform the injection of plastic melt into the
mold. The advantages of a two-stage injection unit include more uniform
plastication of material, reduced wear on the screw and barrel, and the
potential for higher injection pressures. The primary disadvantages are
higher unit cost and material carryover from shot-to-shot that can affect
part quality. More specifically, when thermoplastic material is maintained
in a fluid state (above melt temperature) for an extended period, its
properties will degrade to varying extents depending on the type of
material, the temperature of the melt and the time it is held at the
elevated temperature. The construction of the accumulator and internal
piston generally determines how much material remains in the accumulator
after the shot is injected into the mold.
In either type of system, the injection and extruder functions each require
an associated drive apparatus in the injection unit. In hydraulic
machines, movement of the screw for the injection function is typically
performed by one or more hydraulic cylinders, while the rotation of the
feed screw for extruder run is normally accomplished by a hydraulic motor.
More recently, electric motors combined with mechanical systems have been
used as the direct power source for reciprocating screw injection units.
Notably, these prior art electric systems have used separate motors for
each function; i.e., one motor for rotating the feed screw and a second
motor in combination with a mechanism, such as a ball screw, to convert
rotary motion into the linear movement required to move the screw forward
for injection.
Accordingly, as is typical when new technology is applied to existing
products, the effort has been to maximize the execution of the previous
injection system technology so as to limit risk and retain product
identity. This is especially true in all-electric injection molding
machine design where hydraulic motion control has been replaced with
electromechanical motion control. As a result of this limited design
approach, many important advantages of electric variable speed motor
drives have not been realized in their application to injection molding.
For example, it is generally known that the hydraulically driven
reciprocating screw injection unit design has a shot size consistency and
repeatability capability of approximately .+-.0.2%, due to hydraulic
system fluctuations mentioned above and inconsistency of the non-return
valve at the end of the screw (the non-return valve is a necessary
component to the proper functioning of the reciprocating screw design).
Given that all of the all-electric machines in the market today have a
reciprocating screw, the potential for reducing shot size variation has
been limited to the improvement of positioning repeatability of screw
forward axis alone.
It is well established that simply replacing hydraulic drive trains with
electromechanical drive trains provides significant, measurable
improvement in repeatability, stability, and accuracy of the driven
device. This is a result of reducing the number of components in the drive
train, elimination of inherent variations in the hydraulic fluid as a
function of temperature, viscosity changes due to ultimate chemical
breakdown of the oil itself, eventual increasing concentration of
contaminants, and so forth. However, while simply replacing the hydraulic
drive train components with servo-electrical/mechanical components
provides desirable performance improvement, the full potential improvement
has yet to be realized.
The potential for improvement is particularly evident in reciprocating
screw injection units having relatively large shot capacity. While the
increased shot size is relatively simple in reciprocating screw hydraulic
machines, the substitution of electric motors and ball screws for
hydraulic cylinders ultimately becomes impractical due to the excessive
cost of the large ball screws required (to reciprocate the screw for
injection). Although the size of the ball screws can be reduced by using
two screws in tandem, the costs for the screws and associated components
remain excessively high. In addition, the construction of electric
reciprocating screw injection units that have capacities to match the
range of hydraulic units available would require ball screws of sizes that
are untested and, in fact, exceed current manufacturing capabilities.
In addition, the processing requirements for injection molding commercially
significant plastics materials involve injection pressures of at least
15,000 psi, and frequently up to 30,000 psi. Given that availability and
cost of ball screws are more affected by diameter rather than length, ball
screws in excess of six inches in diameter are virtually unavailable in
commercial quantities--which has severely limited the advance of
all-electric designs beyond about 32 ounces shot capacity. For example, a
typical 100 ounce shot capacity hydraulic injection unit would have a
reciprocating screw of about 4 inches in diameter to generate 20,000 psi.
An all-electric (reciprocating screw) equivalent would need a ball screw
far in excess of six inches in diameter to carry the load. In fact, the
largest commercially produced all-electric injection unit in the world
today uses two ball screws 6.5 inches in diameter to support the load
requirements of a 3.5 inch diameter reciprocating screw operating to
inject up to 77 ounces of melt at a maximum injection pressure of 22,000
psi.
Ball screw performance and durability suffer in reciprocating screw
injection applications. To get optimum useful life from the ball screw,
minimum levels of ball circulation and lubrication circulation must be
accomplished. However, the reciprocating screw design is limited to
relatively short injection stroke, because longer strokes induce
unacceptable plastic processing variations that result from the decreasing
effective screw length to diameter ratio (L/D) as the screw retracts while
building the shot volume for injection. By current standards, rarely does
the injection stroke exceed five times the screw diameter in a
reciprocating screw design. Furthermore, prior art (hydraulic) two-stage
injection units have adhered to roughly the same ratio for the stroke and
diameter of the accumulator piston.
Typically, the size of the "shot" processed in most reciprocating screw
injection units would probably be about 25% of the maximum. (This results
from the fact that the screw is sized by plasticizing requirements rather
than shot capacity.) Using the 25% limitation for purposes of
illustration, in a reciprocating screw, all-electric injection unit,
maximum ball screw travel would likely be limited to one screw diameter or
less for a majority of the machine's service life. Ball screw leads are
typically one-fourth to one-half the diameter of the ball screw and are
usually designed to have at least three complete thread revolutions under
load. For example, if an injection unit is traversing one screw diameter
for injection, and the injection axis ball screw is twice the diameter of
the injection screw, the loaded balls in the mechanism never fully
circulate to unloaded positions and some of the unloaded ball do not move
to a loaded position. This results in uneven wear of the components and
the natural lubrication that would occur from complete circulation of the
balls must be supplanted with frequent, external lubrication. Accordingly,
ball screw life in a reciprocating screw injection unit is less than it
would be in an application where there is full circulation of the balls.
It should be noted that the relatively large diameter and short stroke of
the reciprocating screw injection unit facilitates high speed injection;
however, a high torque motor is required to produce the desired injection
pressures. Since horsepower is a function of the product of motor torque
and RPM, the high torque requirement means that high horsepower motors are
required to drive the injection mechanism,
Another consideration is that the floor space occupied by an injection
molding machine has become an increasingly important criteria. As the
resources once available for facilities are diverted to other assets to
increase productivity, the length, width and height of a machine has
become increasingly important consideration among competing machine
designs. In all-electric machines, the injection ball screw is most
advantageously arranged in line behind the injection piston. In the case
of the reciprocating screw, the plasticizing screw is the injection
piston, and already has a length that is fifteen to thirty times its
diameter because of plasticizing requirements. Since it generally
desirable to lengthen injection stroke as much as possible, positioning a
ball screw in-line with the plasticizing screw for the injection stroke
results in a machine of undesirable overall length.
Besides the need for increased capacity in electric injection units, there
is potential for improvement in durability, repeatability, stability, and
accuracy of the driven device, as well as a reduction in overall length of
the machine, if a way can be found to overcome the obstacles presented by
limiting application of electro-mechanical technology to reciprocating
screw injection units. Furthermore, in order to take full advantage of the
desired improvements, a new construction for an electric injection unit
should not introduce the material carryover problems typically associated
with prior art two stage injection units.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a drive
apparatus for an all-electric injection unit that is simple in
construction, is more durable, improves shot accuracy and effectively
enables increased shot capacity. It is a further object of the invention
to apply variable speed electric drive technology to two stage injection
units in a way that improves operating efficiency and reduces material
carryover with respect to prior art systems.
In accordance with these objectives, the invention is directed to a
two-stage, all-electric injection unit in which the feed screw is used
primarily for plastication and a melt accumulator is used for injection.
Preferably, the feed screw is capable of rotation only and does not
reciprocate, which will reduce the overall length of the machine. The
injection of the plasticized material is accomplished by a separate melt
accumulator having a plunger with a helical flight that is reciprocated by
an electrically driven linear actuator, such as a ball screw mechanism.
Preferably, there are separate motors for rotation of the feed screw and
operation of the accumulator, allowing the drive system for the plunger to
also provide rotation of the plunger by means of a one-way clutch
interposed between the ball screw and plunger. The disclosed two-stage
construction is particularly suited for large capacity injection units
(greater that 80 ounces) where the large ball screws needed to reciprocate
the feed screw in the prior art result in excessive costs.
Although two-stage injection units have been used on hydraulically powered
injection molding machines for many years, they have not been used on
electric machines since the advantages provided by two stage units on
hydraulic machines have been largely accomplished by the application of
all-electric drive technology to reciprocating screw units. The invention
of the two-stage electric injection unit goes beyond the apparent
advantageous and economic use of standard injection unit components that
has occurred in the prior art. In the disclosed embodiment, the invention
enables performance capabilities that are presently unattainable with
existing hydraulic or electro-mechanical injection unit technology.
By optimizing the length of stroke and diameter of the accumulator piston,
the invention achieves important advantages of an all-electric machine
design that have not been previously realized. Since the diameter of the
piston dictates the load carrying requirements for the mechanism that
converts the rotary motion of the motor into linear motion for the piston,
larger shot capacities can be accomplished with the two-stage design by
providing increased length of stroke at relatively small piston diameters.
For example, a two-stage electric injection unit according to the present
invention could have an injection (accumulator) capacity of 150 ounces
(2.75 inch diameter piston, 46 inch stroke) and be capable of operating at
20,000 psi with a 5.5 inch diameter ball screw (which is commercially
available). This diameter of ball screw in prior art all-electric
injection molding machines would typically have a shot capacity of only
about 30 ounces (2.75 inch diameter screw, 9 inch stroke). Accordingly,
the invention expands the shot capacity by about five times without adding
the risk, cost, and space requirements for the larger ball screws required
for a reciprocating screw design.
Further advantages of lengthening injection stroke for a desired shot
capacity include increased ball screw performance and durability. In
particular, the use of an accumulator in the two-stage design provides the
ability to optimize the shot cylinder diameter (of the accumulator)
independent of the plasticizing screw diameter. Thus, the system can be
designed to provide sufficient traverse of the ball screw mechanism to
improve loading and circulation of the balls, improving lubrication and
increasing the service life. The smaller diameter means that less
horsepower is required for a given shot capacity. Although the ratio of
the stroke to piston diameter might decrease to around 10 for high speed
injection, the smaller diameter facilitates injection at desired pressure
levels with a lower horsepower motor.
With the described construction for the accumulator and plunger, material
carryover is essentially limited to only one cycle. When thermoplastic
melt is fed into the accumulator by the extruder, the ball screw in the
plunger drive mechanism is rotated to control the rearward movement of the
plunger, and thus control the back pressure on the melt. The ball screw
rotation is imparted to the plunger by means of a one-way clutch. More
specifically, the thermoplastic melt enters the accumulator at a point
where it wipes away melt remaining in the flight of the plunger from the
previous shot. This flow of new melt moves the carryover material in front
of the plunger and at the end of the accumulator where it will be the
first out in the subsequent shot. This virtually eliminates material
carryover thereby minimizing material degradation and greatly reducing the
time requires to purge one color of material and change to a new color of
material.
The all-electric two stage injection unit design of the present invention
also solves the problem of excessive machine length. In particular, the
plasticizing screw length is disassociated from the injection stroke and
corresponding ball screw length. This is accomplished by independent
support of these two elements (plasticizing screw and ball screw) on
different center lines, thus providing a machine that is compact in length
without adding additional width or height.
Another advantage of the disclosed two-stage injection unit configuration
is that it enables the extruder screw to be gear driven rather than belt
driven. The belt and pulley systems are somewhat limited because of the
strength of belts or the number of belts required to deliver torque to
larger plasticizing screws when they are driven off-center. Moving the
injection function away from the plasticizing screw axis allows access to
hard-couple a mechanical speed reduction gear box to the screw. This is
not possible in an all-electric reciprocating screw injection unit where
the injection ball screw is in line with the plasticizing screw.
The two-stage all-electric design of the present invention makes possible
all-electric injection units with shot capacities far exceeding those of
conventional, all-electric reciprocating screw units. As such, electric
two stage injection units of the present invention have greater potential
machine applications by providing benefits and performance gains relating
to improved repeatability, material stability, and shot size accuracy.
More particularly, the two-stage injection unit offers further advantages
over the prior art by eliminating the need for a non-return valve at the
end of the screw, being able to separate the shot size from plasticizing
requirements, and allowing the use of a smaller diameter injection piston
(as compared to the screw diameter) for more precise control of shot size.
Overall, the present invention provides an all-electric injection unit
having enhanced capabilities, including increased shot capacity, improved
shot control, minimal carryover and faster color changes, when compared to
prior art electrically driven injection systems.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a fragmentary, side elevational view of an injection molding
machine having a two-stage electric injection unit according to the
present invention.
FIG. 2 is an enlarged view, partially in section, of the two-stage electric
injection unit of the molding machine illustrated in FIG. 1.
FIG. 3 is a more detailed, enlarged view, of the injection unit illustrated
in FIG. 2, focusing on the melt accumulator and mechanical drive elements.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention relates to a two-stage electric injection unit 14 for
an injection molding machine; as such, it will be described in the context
of a typical machine. Since the general structure and operation of
injection molding machines are well known, those aspects of the apparatus
which are different or take on a new use with respect to two-stage
electric injection will receive primary emphasis.
The apparatus of the present invention is used in conjunction with an
injection molding machine 10, as shown in FIGS. 1 and 2. The general
configuration of the molding machine 10 includes a conventional clamp unit
12, and a two-stage electric injection unit 14, both of which are mounted
on an elongated support or base 16. The components of the injection unit
14 are specifically designed to implement electric motor drive technology
in a two-stage injection unit. Preferably, the primary elements are an
electrically driven extruder 18 and a melt accumulator 20. The extruder 18
is intended for continuous plasticizing and, therefore, has a
non-reciprocating feed screw 30 (see FIG. 2). If desired, however, the
concepts of the present invention can also be applied to a two-stage
injection system with a reciprocating feed screw.
As is generally known in the art, material is supplied to the extruder in
any convenient manner, such as by a hopper 24. The rotational power for
the screw 30 is also provided in a conventional manner, as by an electric
motor 26, connected to a speed reduction gearbox 28 that drives the screw
30. Since the movement of the screw 30 is rotational only, the drive
system is greatly simplified over the injection units having a screw which
must also reciprocate.
The accumulator 20 is essentially a variable volume reservoir by virtue of
a cylindrical barrel 32 containing a plunger 34 that is capable of both
rotational and linear movement within the barrel 32. Note that the end of
the plunger 34 has a helical flight 36. The relative size of the barrel 32
and plunger 34, as well as the stroke of the plunger 34, will vary
according to the quantity of melt required to fill the mold. In the
constriction of melt accumulator 20, it is desirable to configure the
end-shape of the barrel 32 and plunger 34 in a way that minimizes the
amount of resin remaining in the barrel 32 when the plunger 34 is fully
extended, as will be more fully discussed later.
By optimizing the length of stroke and diameter of the plunger 34 important
advantages of an all-electric machine design can be realized. The diameter
of the plunger 34 dictates the load carrying requirements for the ball
screw that converts the rotary motion of the motor into linear motion for
the plunger 34. However, the larger shot capacities can be easily
accomplished with the two-stage design by providing increased length of
stroke at relatively small diameters. For example, the disclosed two stage
design yields an injection capability of at least 150 oz. at a 2.75 inch
diameter plunger 34 that can operate at 20,000 psi with a 5.5 in diameter
ball screw; in contrast, prior art all-electric injection molding machines
with these design criteria would typically have a shot capacity of only
about 30 oz.
In sizing the components of melt accumulator 20, the advantages of the
invention are more fully realized when ratio of the full stroke of the
plunger 34 to the diameter of the plunger 34 (this criteria is similar to
the L/D of a reciprocating screw) is five or higher; the range of ten to
fifteen for this ratio is believed to be particularly advantageous. This
configuration of the accumulator 20 would enable use of commercially
available ball screws, while providing a longer stroke (improving shot
size accuracy and repeatability) at higher injection pressures.
The outlet of the extruder 18 connects to accumulator 20 via a suitable
conduit 42. At a convenient point between the extruder 18 and the inlet 40
to the accumulator 20, a ball check valve 46 or other suitable non-return
device is provided to control the direction of the flow through conduit
42. When the accumulator 20 is activated to inject plastic into the mold
cavity and maintain pressure during pack and hold, the check valve 46
prevents a back-flow of melt into the extruder 18 due to the pressure
differential. The outlet of the accumulator 20 is connected to the
injection mold (not shown) via a suitable nozzle 56.
The plunger 34 of accumulator 20 is preferably actuated by an
electromechanical drive assembly 60, see FIGS. 2 and 3. The drive assembly
60 preferably includes a ball screw 62, a ball nut 64 with support housing
66, a variable speed electric motor 68 and a motor support 70 that allows
for linear movement of the motor 68. More specifically, the ball nut 64 is
preferably carried within support housing 66 and is restrained from
rotation by its attachment to housing 66 through suitable means, such as a
load cell 76. The driven end of the ball screw 62 connects to the motor
shaft 78; the opposite end of the screw 62 connects to the plunger 34 of
the accumulator 20 by means of a coupling 72. Preferably, the coupling 72
includes a one-way clutch 86 that allows the ball screw 62 to rotate
freely with respect to the plunger 34 during injection (clockwise rotation
of screw 62) to transmit efficiently linear (horizontal) force from the
ball screw 62 to the plunger 34 without adversely affecting the melt
contained in the accumulator 20. However, reverse (counter-clockwise)
rotation of the ball screw 62 engages the one-way clutch 86 causing the
plunger 34 to rotate within the cylinder 32.
Since the shaft 78 of the motor 68 attaches directly to the ball screw 62,
the motor 68 must be able to reciprocate back and forth as the ball screw
62 is used to move the plunger 34. Accordingly, the support 70 for the
motor 68 is configured to provide stability for the motor 68 while
allowing it to move linearly in a direction parallel to the movement of
the plunger 34, as indicated by arrow A. Of course, it is also conceivable
that the connection of motor shaft 78 to ball screw 62 could be
accomplished via an elongated spline coupling, allowing a stationary
mounting of the motor 68 at a position more rearward from that shown.
A cycle of operation of the injection molding machine 10, incorporating the
two-stage injection unit 14 of the present invention will now be
described. The feed screw 30 is rotated within barrel 38 by the extruder
motor 26 to begin plastication of the material that will be supplied as
plastic melt to the accumulator 20. The rotation of the screw 30 builds
pressure at the end of the screw 30, moving (opening) the ball check valve
46 and causing material to flow through the conduit 42 and into the
accumulator 20.
The inlet 40 of accumulator 20 is positioned so that melt flowing into the
barrel 32 will pass over the flight 36 at the end of plunger 34. The
incoming melt will flow along the flight 36, cleaning out melt carried
over from the previous shot and moving it toward the outlet end of barrel
32, causing the pressure in the accumulator 20 to build. When the pressure
of the plastic melt reaches a certain level, it will begin to force the
plunger 34 rearwardly, thereby moving the ball screw 62 and motor 68
toward the rear of injection unit 14 (support housing 66 remains
stationary), see FIG. 3. The rearward movement of plunger 34 applies a
force to ball screw 62 through coupling 72, causing ball screw 62 to move
likewise to the rear; as the ball screw 62 moves through ball nut 64 it
rotates in a reverse (counter-clockwise) direction. This reverse rotation
of the ball screw 62 is imparted to the plunger 34 via engagement of the
one-way clutch 86. The rotation of plunger 34 further aids in cleaning
carry-over material from the flight 36 by enhancing the wiping action of
the inflow of new melt.
If desired, the rate of rearward movement of the plunger 34 (and ball screw
62) can be controlled by the motor 68. In particular, the motor 68 can be
used as a brake to impede the rotation of ball screw 62, which slows the
rearward movement of the plunger 34, thereby increasing the back pressure
of the plastic melt. Alternatively, the motor 68 can be used to speed up
the rotation and rearward movement of the ball screw 62, which increases
the rate at which the plunger 34 moves back, thereby decreasing the back
pressure of the melt. In either case, the rotational speed of the ball
screw 62 is imparted to the plunger 34 by clutch 86.
The extrusion function is complete and rotation of the feed screw 30 is
stopped when a sufficient charge of plastic melt is accumulated in front
of the plunger 34 in the accumulator 20, as required to fill the cavity of
the mold. Concurrently with the extrusion function, the clamp unit 12 has
been operated to close and build pressure on the mold that will receive
the plastic melt.
To initiate the injection function, the motor 68 is rotated in a clockwise
direction causing the ball screw 62 to advance through ball nut 64 which
is constrained by support housing 66. The translational (linear) movement
of screw 62 is imparted to the plunger 34 housed in the accumulator 20.
However, the rotation of the screw 62 is not imparted to the plunger 34
since the one-way clutch 86 is disengaged when the screw 62 rotates in the
clockwise direction. Except for an assembly having a stationary motor
mounting, as described above, the motor 68 will also have translational
movement (since it is part of the same assembly) along with the ball screw
62 as the plunger 34 is moved linearly in the cylinder 32 of the
accumulator 20.
The forward movement of the plunger 34 causes the accumulated plastic melt
to be forced through the nozzle 56 and into the mold cavity. The injection
pressure generated by movement of the plunger 34 moves the ball check
valve 46 to a position that prevents transfer of the melted resin into the
extruder 18. After the bulk of material is transferred into the mold
cavity, the injection accumulator 20 initiates pack, then hold, to
maintain the proper pressure on the material until the molded part is
properly formed. When the injection accumulator 20 reaches the "hold"
portion of the cycle, it has emptied itself of material. In other words,
the injection of plastic melt is accomplished by applying sufficient force
to move the plunger 34 rapidly forward in the cylinder 32, forcing the
melt to flow through the outlet of the injection accumulator 20, on
through the nozzle 56, then into the mold. This approximate point in the
cycle can be identified by the configuration shown in FIG. 2; the plunger
34 in the accumulator 20 is fully forward in the barrel 32, having
completed the injection function.
As part of the injection process, it is highly desirable to avoid "dead"
spots in the material flow path where plastic melt can remain stationary
through repeated cycles, allowing it to degrade, possibly later mixing
with good material and injected to form a poor quality part. Accordingly,
a mating configuration between the end of the plunger 34 and the outlet of
the barrel 32 will minimize the amount of material remaining in the
accumulator 20 after the shot is completed. The only significant
carry-over material is in the flight 36 which is wiped clean by the inflow
of new melt and rotation of the plunger 34 as the subsequent shot
accumulates (and is injected into the mold during the next cycle of
operation).
After sufficient hold/cool time, the pressure held by the injection
accumulator 20 is released during mold decompress, which may include a
slight retraction of the plunger 34. The clamp 12 operates to open the
mold, eject the part(s), then re-close to begin a subsequent cycle. As
soon as the pressure by plunger 34 is released, the injection unit 14
starts rotation of the feed screw 30 to initiate the extrusion function as
described previously and begin another cycle of operation.
While the invention has been illustrated in some detail according to the
preferred embodiment shown in the accompanying drawings, and while the
preferred embodiment has been described in some detail, there is no
intention to thus limit the invention to such detail. On contrary, it is
intended to cover all modifications, alterations, and equivalents falling
within the spirit and scope of the appended claims. For example, although
the drive couplings are generally described as belts and pulleys, other
mechanical couplings, such as suitable gearing, can be used to perform the
same function. In addition, other systems or mechanisms can be used to
impart linear motion to the accumulator plunger 34; such as, a roller
screw and nut in place of the ball screw and ball nut as described.
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